<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
     div[class^="col"]{
        height: 20px;
        border: 1px solid #333;
     }
    </style>
</head>
<!-- 栅格的原理 把容器等分为12等分
    md 电脑@media（min-width:992px）{}
    sm 平板@media(min-width:768x)and(max-width:992px){}
    xs 手机-->
    <!-- 在平板上 每个div 宽50% -->

<body>
    <div class="container">
        <div class="row">
          <div class="col-md-3 col-sm-6 col-xs-8">

          </div>
          <div class="col-md-7 col-sm-6 col-xs-4">

          </div>
          <div class="">

          </div>
        </div>
    </div>
</body>
</html>